A fuel cell is a device for converting chemical energy into electricity. An acid fuel cell comprises an anode, a cathode and an acid electrolyte held within a porous nonconducting matrix between the anode and the cathode. The anode and cathode each comprise a porous substrate and a porous catalyst layer supported on the porous substrate. Phosphoric acid is typically used as the acid electrolyte because it is nonvolatile and stable in both hydrogen and oxygen at the operating temperatures of an acid fuel cell.
Conventional acid fuel cells operate at temperatures in the range of 150o C to 250o C at pressures between 1 and 8 atmospheres. A hydrogen containing gas is fed to the anode of the cell and an oxygen containing gas is fed to the cathode of the cell. At the anode the hydrogen is electrochemically oxidized and gives up electrons according the reaction: EQU H.sub.2 .fwdarw.2H.sup.+ +2e.
When the anode and cathode are connected through an external circuit the electrons so produced travel from the anode to the cathode, where they react electrochemically according to the reaction: EQU 1/2 O.sub.2 +2H.sup.+ 2e.sup.- .fwdarw.H.sub.2 O.
A flow of hydrogen ions through the acid electrolyte completes the electrical circuit.
Conventional gas diffusion electrodes used in acid fuel cells consist of a catalyst layer supported on a porous, conductive substrate. The catalyst layer typically consists of a mixture of supported catalyst particles, e.g. a finely divided noble metal dispersed over a porous conductive support and a porous hydrophobic binder, e.g. sintered submicron size particles of polytetrafluoroethylene.
The electrochemical reactions occur at the surfaces of the supported catalyst particles. The support material forms a network of electrolyte filled channels and the porous hydrophobic binder provides an interpenetrating network of hydrophobic channels for gas transport. The reactant gases gain access to the catalytic surfaces and gaseous products escape from the catalyst layer through the network of hydrophobic channels.
The performance of an acid fuel cell is limited at high current densities by phenomena termed "flooding" and "pumping".
"Flooding" refers to the penetration of electrolyte into hydrophobic regions of the catalyst layer which should contain only gas. This misplaced liquid hinders and may totally obstruct the supply of reactant gas to local regions of the catalyst. As a result there is an increase in electrode polarization as the non-flooded regions of the electrode are forced to carry more current. The process is self-propagating and will eventually lead to cell failure. In fuel cells having an acid electrolyte the flooding phenomenon is most prevalent at the cathode i.e., at the water-producing electrode.
The term "pumping" refers to the bulk movement of electrolyte from one side of the cell to the other. For example, in the case of fuel cells with phosphoric acid as the electrolyte it results from the electromigration of the non-electroactive phosphate anions towards the anode. At high current densities sufficient pressure can be built up in the anode to force electrolyte out of the back of the electrode. Accumulation of electrolyte on the gas side of the electrode restricts the supply of reactant and can lead to severe concentration polarization.
The conventional approach to the problems of electrolyte intrusion into gas electrodes has been to make the catalyst layers of the electrodes more hydrophobic. The catalyst layer may be made more hydrophobic by increasing the relative amount of hydrophobic polymer in the layer or by increasing the temperature or the process time during the sintering of the electrode. These strategies have achieved only limited success.
What is needed in the art is a way to reduce the flooding and pumping phenomena to provide fuel cell electrodes that may be operated continuously at high current density.